This paper proposes the concept of how machinery condition monitoring can be taken to the next level, through micro-sensing of tribological phenomena occurring between contacting surfaces. By considering wear transitions and wear rates it is possible to distinguish between benign and potentially harmful wear scenarios. By measuring the tribological phenomena associated with these conditions, it should then be possible to determine with greater accuracy the health of a machine at any point in its life. For this approach to succeed, it is necessary to develop a comprehensive and holistic monitoring strategy and target sensing technologies for the key wear factors. The paper has two main sections. Firstly, tribological phenomena and the onset of wear which sets out why and what needs to be monitored. The factors influencing the wear process are grouped into three key areas: lubricant condition, tribo-pair condition and operating condition. Through a critical and comprehensive review of developing and state-of the-art tribo-sensing, the second section identifies the potential technologies for monitoring or measuring the physical parameters within these three groupings and thus sets out how the next generation of machine condition monitoring will need to evolve in order to achieve early wear detection and the related benefits. •The factors influencing wear are grouped into three areas: lubricant condition, tribo-pair condition and operating condition.•In-situ measurements of tribological phenomena provide an early indication of wear, ahead of impending failure.•We use the derivative of instantaneous wear rate (dIWR) to distinguish between natural and induced wear transitions.•Wear transitions and rates distinguish benign and potentially harmful scenarios to determine the health of a machine.•Reviews monitoring tribo-phenomena, identifies existing capability and gaps and how current technologies address these gaps.